1. Field of the Invention
The present invention generally relates to image forming apparatuses.
2. Description of the Related Art
Conventionally, a switch-back method and a one-pass method have been known as methods for forming images on both surfaces of a transferring medium such as a copy paper in an electrophotographic image forming apparatus. In the switch-back method, after an image formed on a surface by a transferring part is fixed by a fixing part, the transferring medium is reversed (switched back) and then transferring and fixing are performed on the opposite side surface (rear surface) so that an image is formed on the opposite side surface. On the other hand, in the one-pass method, after images are transferred onto both surfaces by a both-surfaces transferring part, the transferring medium is provided at the fixing part. Hence, the images can be formed on both surfaces of the transferring medium without switching back the transferring medium.
The one-pass method is superior to the switch back method in the following points. In other words, in the one-pass method, cost increases due to providing the complex switch back mechanism for reversing the transferring medium can be avoided. Furthermore, in the one-pass method, generation of jams due to a long interval for image forming by the switch back method and switching back of the transferring medium which is curled by heat of the heating part can be avoided.
The following apparatuses have been known as examples of an image forming apparatus for one-pass both-surfaces printing, the apparatus not requiring a both-surfaces reversing mechanism or a both-surfaces conveyance path.
For example, Japanese Laid-Open Patent Application Publication No. 2002-189358 describes an image forming apparatus having a first intermediate transferring belt and a second intermediate transferring belt. A transferring medium such as a printing paper can be transferred by the second intermediate transferring belt. A toner image on the first intermediate transferring belt can be transferred by the second intermediate transferring belt.
Japanese Laid-Open Patent Application Publication No. 2-259670 describes an image forming apparatus whereby a pair of photosensitive members for individually forming toner images having different electrostatic charge properties is used so that the toner images are transferred onto both surfaces of a transferring sheet.
Japanese Laid-Open Patent Application Publication No. 9-211900 describes an image forming apparatus whereby toner images are formed on both surfaces of a transferring sheet by using an individual intermediate transferring belt which holds a rear surface image of each color and a contact and un-contact mechanism between the intermediate transferring belt and a photosensitive member.
As discussed above, although the one-pass image forming apparatuses have been suggested, there are points to be improved such as the speed of image forming, miniaturization or low price of the apparatus, and the quality of a formed image.
For example, in the image forming apparatus described in Japanese Laid-Open Patent Application Publication No. 2002-189358, after the toner image on the rear surface is transferred onto the second intermediate transferring belt, the transferring medium is stacked on the intermediate transferring belt where the toner image is transferred so as to be carried. Accordingly, the toner image of the rear surface on the intermediate transferring belt and the transferring member are rubbed so that quality degradation of the image may occur. Furthermore, conveyance of the toner image by lengths of two intermediate transferring belts until the rear surface is transferred is necessary so that the printing time becomes long.
In addition, in the image forming apparatus described in Japanese Laid-Open Patent Application Publication No. 2-259670, a pair of photosensitive members or a developing unit is necessary for forming the toner image on a surface or a rear surface. Hence, there is an increase of cost due to the increase in the number of components or the large size of the apparatus.
Furthermore, in the image forming apparatus described in Japanese Laid-Open Patent Application Publication No. 9-211900, there are four intermediate transferring belts for black (Bk), magenta (M), cyan (C), and yellow (Y) colors. Therefore, there is a problem where color adjustment is difficult, in addition to an increase in cost due to an increase in the number of components or in the large size of the apparatus.